Mark "deceased" when prompted by the FAFSA to report the father's income. No other documentation to prove the death is necessary.
Use the tax returns of the surviving parent, no matter how uninvolved that parent was in supporting the child. However, if the surviving parent has never claimed the student as a dependent, the student can petition the school for dependency override. The student can then file the FAFSA without any parental information. Even if the the surviving parent does not claim the student, the parent's tax documents are needed to complete the FAFSA. However, the student can petition the school to be declared independent. This has to be done after the student has been accepted to a university.
Explain in the allotted space on the FAFSA any financial burdens that having a deceased father has placed on the student.
File as an independent student if both parents are deceased. The expected family contribution will be set at zero if the student earns less than $50,000.